Agar-agar is a mixture of Polysaccharides (agarose+agaropectine) of a high molecular weight between 40.000 and 200.000. Its chemical structure is a disaccharide composed of .beta., D, galactopyranose (1.fwdarw.3) linked to (3-6) anhydro, .varies.-L galactopyranose (1.fwdarw.4). It can be also functionalized by sulphate grouping, metoxyl and pyruvat. The position and the proportion of these groupings vary according to the red seaweed used for the extraction (Gelidium, Gracilaria, Pterocladia etc. . .)
The main property of agar-agar is dissolution in hot water at reflux or in autoclave under pressure after several minutes of boiling. While cooling, this solution transforms to gel and liquifies if it is heated again.
This thermoreversible gel is characterized by a high hysteresis. The gelation temperature is 36.degree. C.+4.degree. C. or 40.degree..+-.4.degree. C. respectively for agar-agar extracted from Gelidium or Gracilaria. The melting temperature is 85.degree. C..+-.5.
Agar-agar is obtained by extraction in water from red seaweed species like Gelidium, Gracilaria Pterocladia, Anfheltia, etc. The extraction consists in boiling in autoclaves under pressure or in open tank. The obtained extraction liquor contains about 2% agar-agar. It is filtered then gelated. The obtained gel is thus dehydrated either by mechanical pressing through filtering cloth, or by freeze-thawing at minus 20.degree. C. that precipitates agar. These two treatments allow to obtain gels of agar-agar with moisture contents of 60 to 90%, that we call dehydrated agar-agar gel (I) in the text that follows. They are dried and grinded in order to obtain an agar-agar powder with a moisture of less than 18% that we call classical agar-agar (II) in the text that follows.
A person of skill in the art knows that the agar-agar produced by these two procedures independent of seaweed species dealt with, has characteristics of dissolving that require boiling the agar powder in water for a dozen minutes to get a complete dissolution and a good gel strength. This property makes difficult the use of the agar-agar obtained by these 2 procedures in the applications it is designed for as a gelling, thickening and texturing agent.
Many methods were described to solve this problem of dissolution. U.S. Pat. No. 3.119.697 dated Jan. 28, 1964 describes a treatment that consists in heating a concentrated agar-agar solution of 5 to 8% to which is added a quantity of crystallization controlling agent such as carbohydrate, polyol or a known dispersant. This solution is gelated, then dried in hot air. The obtained product dissolves in water at 60.degree. C. but since it is a mixture of products, it can be used only in restricted and focused food applications.
G. Pappes' paper (Journal of Food Sciences 52 (2), 467-470, 1987) describes a procedure that consists in drying directly in a rotary vaccuum drier agar-agar solutions of 1 to 3% concentration and in recovery of the dried film and grinding it. This procedure allows obtaining an agar-agar with complete dissolution properties at 55.degree. C., but induces high productions costs. The same principle is asserted by the Japanese patent No. 61025470 dated Apr. 2, 1986. Other patents present other treatment possibilities as described by European patent No. 0122289 dated Oct. 12, 1993 consisting of a treatment of agar-agar powder with non-equilibrium plasma. Japanese patent No. 1153067 dated Jun. 15, 1989 consists in treating by quick freezing an agar-agar gel.